Many conventional messaging systems include portable radio communication devices carried by subscribers and at least one fixed base station for transmitting messages to the radio communication devices for subsequent presentation to the subscribers. In some of these systems, one or more of the radio communication devices may be transceivers capable of transmitting signals as well as receiving messages from the base station. These systems are described as two-way messaging systems.
In two-way messaging systems, a portable transceiver can signal a base station for a number of reasons. By way of example, the portable transceiver could transmit a signal to the base station to deliver a message, to acknowledge reception of a message, or to indicate to the base station that the portable transceiver is located within the coverage area of the base station.
Currently, one-way messaging systems, e.g., paging systems, are typically allocated a frequency channel, for example, 25 kHz. To add two-way messaging capability to such a system, portable transceivers require limited throughput to the base station, e.g., 100 bits/sec. Additionally, to maximize battery life, it is desirable for the portable transceivers to use low power levels when transmitting. However, it is then necessary to transmit at a very low rate in order to balance the range between the low power inbound channel and the high-rate, high-power outbound channel. If the low-rate inbound signals are multiplexed into the channel, a plurality of the portable transceivers could share a single channel.
One possible method for multiplexing a set of narrowband inbound signals is frequency division multiple access (FDMA). However narrowband FDMA has the undesirable property that a great deal of frequency accuracy is required in the local oscillators of the portable transceivers in order to contain each signal within its allotted spectrum. In order to fully utilize the spectrum allocated for FDMA, it would be necessary to have nearly perfect frequency generation in the portable transceivers, which is expensive to implement. To allow for error in the transmitter local oscillators, it is therefore necessary to provide sufficient guardbands between inbound subchannels. However, this results in a waste of spectrum.
Another method for multiplexing a set of narrowband inbound signals is the use of code division multiple access (CDMA). CDMA portable transceivers are simple to implement in comparison to those used in narrowband FDMA systems because multiplexing occurs in the code domain where frequency accuracy is not as critical. In addition, guardbands between subchannels are not required because all subchannels occupy the same spectrum. However, it is well known that CDMA systems are interference limited, i.e., the number of simultaneously transmitting portable transceivers is limited. Furthermore, existing CDMA systems often suffer from the near-far problem. This problem occurs when a fixed base station is unable to detect an inbound spread spectrum signal that is significantly lower in power than other simultaneously received inbound spread spectrum signals. In order to make a CDMA system perform properly, therefore, it is necessary to control the amount of interference, which depends upon the amount of traffic, on the inbound channel. Additionally, it is typically necessary to include complex power control circuitry, which is often expensive and bulky, in the portable transceivers to ensure that all of the transmitted spread spectrum signals are detectable by the base station.
Thus, what is needed is a CDMA system that alleviates the near-far problem while allowing flexibility in the number of simultaneously transmitting portable transceivers. Furthermore, the portable transceivers for use in the CDMA system should not require the addition of complex power or frequency control circuitry.